For the deposition of tin upon a steel sheet or ribbon, mainly acidic solutions containing fluoroborate, phenolsulfonic acid, hydrogen halides or methanesulfonic acid (hereinafter MSA) or mixtures of these acids have been used in the tinplating industry. These acidic electrolytes all have in common that they attack steel in the initial phase of the electrolysis due to their strong acidity. In this initial stage, the thickness of the deposited tin layer is not yet sufficiently high to protect the steel against the acidic medium, thereby iron is dissolved and remains in the solution because it is not deposited from the strongly acidic electrolyte.
Low concentrations of ferrous ions, ranging from about 1 to 5 g/l can be tolerated in these processes. However, concentrations in the order of 10 to 15 g/l Fe adversely affect the process by narrowing the range of operable current densities and by strongly promoting the oxidation of Sn.sup.2+ ions into Sn.sup.4+ ions. A broad range of operable current densities is necessary for an effective tin electrolysis. The formation of tin(IV) oxide is very disadvantageous for the tinplating process, because this insoluble oxide drops out of the solution and is collected as a muddy precipitate on the bottom of the tinplating bath; furthermore it may result in porous tin deposits. Therefore suppressing the formation of insoluble tetravalent tin (SnO.sub.2) is a major concern in any tinplating process. As this formation is strongly enhanced by the presence of dissolved iron, there have been several attempts to separate ferrous ions from tinplating baths.
The Fe content can be kept constant in processes operating with hydrogen halide by adding soluble sodium ferrocyanide to form a ferrocyanide of iron. However, a different approach has to be taken for tinplating baths operating with a polysulfonic acid, an acidic fluoroborate or MSA electrolyte. In such baths, a lowering of the Fe content has been attempted by diluting the bath with fresh electrolyte and discarding part of the old one, or by plating out tin with insoluble anodes, removing iron by ion exchange and then redissolving the tin. Such processes, however, are very expensive.
A process of precipitating iron with organic or inorganic acids has not been successful because the high proton concentration of the electrolyte causes a precipitation of the acids as such or contributes to redissolving the formed iron compound. A chemical separation of iron by means of a pH change has also proven to be unsatisfactory. At a pH value of around 1.2, divalent tin hydroxide Sn(OH).sub.2 precipitates, while iron only precipitates at pH values of around 3. Even in the best case, only mixtures of Sn(OH).sub.2 and an iron salt such as ferrous oxalate were obtained.
The recently published method of iron separation by means of electrodialysis may lead to precipitates of Fe(OH).sub.2 and Sn(OH)2. In general, such a method is laborious and requires well trained personnel and, furthermore it is doubtful whether it is suitable to comply with the high amounts of metal hydroxides which have to be processed. A typical tinplating bath may have a volume of 100,000 to 120,000 liters. Treatment of such amounts of liquid represents a problem of its own which this invention sets out to avoid. For such reason, a method wherein Sn ions, Fe ions and detrimental cations are absorbed from a tinning liquid by passing the liquid through a strong acid cation exchanger such as described in EP-B-0 621 354 is disadvantageous as well. U.S. Pat. No. 5, 057, 290 discloses processes and apparatus for the closed-loop regeneration of spent hydrochloric acid pickle liquors by recovering ferrous chloride from the spent pickle liquors at very low temperatures. However, this patent does not relate to the recovery of tinning electrolytes and is not concerned with the particular necessity of separating stannous and ferrous ions in such an electrolyte.
In view of the above, the present invention wishes to avoid the above-mentioned problems involved with prior art methods for removing Fe ions from tin electrolytes, particularly from electrolytes used as tinplating baths. Moreover, the present invention aims at providing such a separation method which is reliable, inexpensive, provides a good separation performance and can be operated by personnel without specific education. Furthermore, the method should be suitable for the treatment of large amounts of electrolyte, particularly MSA-containing electrolyte. It would be especially advantageous if the method was also suitable for electrolytes containing a mixture of MSA and sulfuric acid.
A further object underlying the present invention is to provide a recovery plant for tinning electrolytes which allows the separation of ferrous ions from iron-containing tin electrolytes on the basis of MSA and, optionally, sulfuric acid.